You'll probably be wise to feel things out with friends or family who have passed the practical tests here though, because there are certain things you have to pass on the practical that you never had to do in the US--backing up certain distances around a corner, certain parking things, not crossing your hands over, etc that an instructor can really help with.
I had driven in the UK too, and felt very comfortable on the roads when I was driving, but I wouldn't have passed the test without the things that I learned from my instructor. I got a good one who didn't make me take tons of lessons as if I were new, but just helped me sort out the things that were different.
Good luck! I'm pretty sure there are some here who do it without instructors, but I think it's worth the expense since the practical is way harder here. Oh, one last thing. If you don't use an instructor's car, you'll need to check on things you need like an extra mirror, L plates, etc. You do have to add a few things to your car, in case you didn't know.